Reliability, build quality, they're just good at making good quality cars. Like another honda.
do you think VW is interested in cash? Automakers do this ALL the time, they source an engine from another automaker if they can't or won't build what they need. The competitors dont mind as its simply a boost to their bottom line.
toyota, mercedes, vw, nissan, chrysler, chevy, ford... almost every company has diesel engines, most companies even have small diesels in other countries. A diesel engine is not hard to make. they could easily make their own, but there are plenty of options for sourcing one as well. you can even look outside the mainstream automotive industry. There are tons of companies that make diesel engines for other uses as well.